This application relates generally to the field of well drilling.
In many cases, the formation pore pressure gradient and the fracture pressure gradient increase with the true vertical depth (TVD) of a well. For each drilling interval, a mud density (mud weight or MW) may be used that is greater than the pore pressure gradient, but less than the fracture pressure gradient, such that a downhole mud, or drilling fluid, pressure lies between the pore pressure and the fracture pressure. In many cases, the difference, also called window, between downhole pore pressure and fracture pressure is sufficient so that the equivalent circulating density (ECD) of the drilling fluid remains within the allowable density window. The ECD, as used herein, is the effective density exerted by a circulating fluid against the formation that takes into account the pressure losses in the annulus above the point being considered. ECD comprises the static mud weight pressure at a depth location in the well added to the pressure losses of the return flow in the annulus between that depth and the surface and then converted to density units. A typical conversion between ECD and pressure at a downhole location isECD (in pounds per gallon, ppg)=annular pressure loss (in psi)÷0.052÷TVD (in ft)+current mud weight (in ppg)  (1)
In some cases, it may be difficult to maintain the ECD within the allowable density window, for example due to an increased annulus pressure drop.
Models and systems for controlling the ECD may use physical and rheological properties of the drilling fluid to calculate various pressure losses in the drilling system. In some cases, the density and rheological properties of drilling fluids are measured manually and reported once, or twice, daily. These properties are then manually entered into the models to generate, at best, spot checks of dynamically changing fluid properties in the system. The accuracy of the models, in real time, is dependent on fluid properties that may have changed substantially since the last fluid property measurement.